I think that Sirius's death, is, uh... how shoud I put it..., multifunctional.
Of course, there's the obious thing, that it will add to Harry wanting to destroy LV. It was like there was this huge bon fire going on inside Harry's mind/heart and then when Sirius dies it's like someone threw a couple gallons of gasoline in the fire!
I also think that Sirius's death was meant to help develop other characters besides Harry. For instance, it really brings Lupin forward. Lupin is now the last marauder, Harry's last link to his father
. Lupin has also lost James and Sirius. He has dealt with loss and rejection. It gives him a chance to be a bigger part of Harry's life, a father figure, a mentor (I'm leaning more towards mentor though). Other characters that it would affect is Neville and Luna. Now they know for sure that Sirius wasn't some crazed escaped lunatic, they saw how he cared about Harry. It also develops Bellatrix Lestrange more, because she's the one who did the killing. Harry can truly hate her now (of course he already hated her for what she did to the Longbottems). Harry may want to kill LV, but now he also wants to deal with Bellatrix. That also adds to his relationship with Neville, they can both hate the same person now, what great dinnertime conversations that will make
! haha, i'm kidding, i'm kidding...
His death also brings that veil into larger play. We saw the veil while the kids were walking around, but now that Sirius has gone through it, it will almost like it's calling out to Harry. Harry will want to talk to Sirius and his parents and such again. It's probably really dangerous (that why its in the DoM), but Harry's dealt with all sorts of dangerous stuff before though, right?